Anticorrosive Performance of Zinc Phosphate Coatings on Mild Steel Developed Using Galvanic Coupling

Abstract

The anticorrosive performance of zinc phosphate coatings developed by galvanic coupling technique on mild steel substrates using the cathode materials such as titanium (Ti), copper (Cu), brass (BR), nickel (Ni), and stainless steel (SS) is elucidated in this study. Thermal and chemical stability tests, immersion test in 3.5% NaCl, ARE salt droplet test, and salt spray test were carried out. The study reveals that the mild steel substrates phosphated under galvanically coupled condition showed better corrosion resistance than the one coated without coupling. The open circuit potential (OCP) of phosphated mild steel panels in 3.5% NaCl was found to be a function of phosphate coating weight and porosity of the coating.

Nano zinc phosphate coatings for enhanced corrosion resistance of mild steel

“…A high charge transfer resistance and low double layer capacitance values were obtained for coatings developed using nano ZnO particles. The impedance studies confirmed that the corrosion behavior of phosphated panels using nano zinc oxide particles is a much more diffusion controlled process and thereby offering a higher corrosion resistance than the normal phosphate coating [16]. As discussed earlier, nano zinc oxide particles increases the surface coverage, uniformity and the thickness of the coating and decreases the porosity which improves the corrosion resistance of the phosphate coatings more than that of the normal zinc phosphate coating [20].…”

“…Salt spray test was carried out by subjecting the phosphated panels to a salt mist of 5% sodium chloride solution in a salt spray chamber (ASTM B 117-03) for a specified period of time (96 hours) [16] . The edges of the substrates were sealed with paraffin wax to avoid the excessive corrosion at the edges.…”

“…As paint pre-treatment coating, the phosphate coating is expected to improve the adhesion of the paint coating and prevents the spreading of underfilm corrosion. The ability to prevent underfilm corrosion is best measured by assessing the spreading of corrosion from the X-scratch made on the phosphated and painted mild steel substrate [ 16 ].The spreading of corrosion from the scribe is less in the case of nano zinc phosphate coatings than normal zinc phosphate coatings on mild steel specimens after 96 hours of salt spray test ( Fig.11). During salt spray test, chloride ions attack the base metal, forming iron (II) chloride followed by hydrolysis and rust formation at the anodic areas (exposed bare steel in the scribed region).…”

“…During salt spray test, chloride ions attack the base metal, forming iron (II) chloride followed by hydrolysis and rust formation at the anodic areas (exposed bare steel in the scribed region). The corresponding cathodic reaction is the reduction of oxygen to form hydroxide ions which dissolve the phosphate at the cathodic areas [ 16 ].…”

“…Also it has been reported that the addition of Ni 2+ into the phosphating bath improves the corrosion resistance of zinc phosphate coatings on the 2024 Al alloy [5,13]. The utility of the galvanic coupling for accelerating low temperature zinc phosphating processes was established recently [14][15][16].Nowadays, nanostructure materials have attracted considerable interest due to their importance in fundamental research and potential wide ranging applications. In the coating industry, the quality of the coatings might be enhanced using nano particles [17][18][19][20].In the present study, phosphate coating was produced by using a phosphating bath consisting of nano ZnO particles instead of normal ZnO particles.…”